We previously identified a novel PTPase in human skeletal muscle by PCR and reported the cloning of the full-length cDNA from a HeLa library. We repeated the Northern analysis using a longer probe and found a strongly hybridizing band of about 4.8 kb in rabbit skeletal muscle. We also found two sequencing errors: one was the omission of a 69 bp EcoR1 repeat (causing the predicted protein to have a 23 amino acid "insert"), and the other was the omission of a single G residue (changing the ORF to now encode a protein of predicted molecular weight 88090). This PTPase has been called PTP-PEST due to the presence of four regions rich in Pro, Glu/Asp and Ser/Thr. PEST domains are often found in proteins with short intracellular half lives. In order to determine if the PEST sequences do alter the half life or have any other regulatory role, the full length protein and a truncated version that contains the entire catalytic domain but lacks all the PEST sequences have been expressed in Sf9 cells using a recombinant baculovirus. In order to determine if insulin alters the enzymatic activity or phosphorylation state of this PTPase, each form of PTP-PEST can now be co-expressed with the insulin receptor. In addition, monoclonal antibodies against PTP-PEST are being generated that can be used to determine protein levels and enzymatic activity in skeletal muscle extracts from insulin sensitive and resistant Pimas. Another approach to indentify the PTPases that are present in skeletal muscle is to resolve (and purify) them by chromatographic methods. We have resolved 4-6 PTPases by ion-exchange chromatography from soluble extracts of rabbit skeletal muscle and various muscle-like tissue culture cell lines. The PTPase that elutes at approximately 0.32 M NaCl from anion-exchange resins has been partially purified and has a molecular weight of about 67 kDa and a specific activity of about 10umol/min.mg using an insulin receptor peptide as substrate.